1. Field of the Invention
The present invention relates to an illumination light color estimating method of obtaining white balance and an image sensing apparatus using the estimating method and, more particularly, to a technique of obtaining a satisfactory image by an electronic still camera or movie/video camera independently of the color temperature of illumination light.
2. Description of the Related Art
Electronic still cameras and movie/video cameras employ an automatic white balance adjusting technique. In the internal circuit of a camera, color difference signals (ER−EY) and (EB−EY) are generated from a signal obtained by photographing an object. To realize automatic white balance adjustment, the color difference signals (ER−EY) and (EB−EY) are integrated over the entire screen. The gains of red and blue signals are controlled such that the integration average values of the color difference signals respectively match predetermined reference levels.
In this automatic white balance adjusting method, however, when a chrominance signal with a high degree of saturation is present, the average integration values change (are affected) by the chrominance signal with a high degree of saturation. For this reason, no white balance is set.
To solve this problem, in Jpn. Pat. Appln. KOKAI Publication No. 1-46393, in obtaining integration average values, slice level A is set for each color difference signal to detect a strong color difference signal. When a strong color difference signal, higher than the set slice level A is present in the color difference signals, the strong color difference signal is replaced with a weak color difference signal, and the replaced color difference signal is integrated.
As a result, the strong color difference signal is replaced with the weak color difference signal. Hence, white balance can be prevented from greatly changing due to the influence of a chrominance signal having a high degree of saturation. However, when a strong color difference signal is replaced with a weak color difference signal, the reproduced color of the image becomes light in accordance with the slice level. Also, when an object having a similar color is photographed, the reproduced color of the image becomes light.
As is well known, when an object is photographed under domestic fluorescent lighting, a greenish image is obtained. This is because light from a fluorescent lamp contains a larger amount of green than an incandescent light source. To set white balance under fluorescent lighting conditions, the slice level for a color difference signal must be shifted to the green side. However, when the slice level is shifted to the green side, green becomes light on a screen in an outdoor photography mode due to the large green component of trees, etc. In addition, white changes to light magenta.
In the above-described illumination light color estimating method for automatic white balance, when a strong color difference signal is replaced to the slice level difference signal, the color on the screen becomes light in a predetermined level. And even when a similar color is photographed, the color on the screen becomes light. In addition, when white balance is set under illumination light of a fluorescent lamp at the time of photographing, white balance shifts at the time of outdoor photographing under a condition of a large green component is present.